This invention relates to a membrane, process and system for removing particles such as virus particles from solutions such as aqueous protein solutions effectively, selectively and reproducibly. More specifically, this invention relates to a composite asymmetric membrane having a specific microstructure which is useful in a process or system for removing virus at a log retention value of between about 3 and 8, i.e., about 99.9% to 99.999999% removal of particles from solution.
Virus represent a potential contaminant in parenteral and other solutions containing a protein which are derived from either whole organisms or mammalian cell culture sources. Currently several chemical and physical methods exist to inactivate virus. These methods are not generic to all virus equally and some operate at the expense of protein activity. For example, heat pasteurization is used in solutions where protein denaturization can be minimized through the addition of stabilizers. In the biotechnology industry, strategies have been adopted that combine several inactivation or removal steps in the downstream process to maximize virus removal capability and protein recovery. The operations used are generally those operations optimized to purify the parenteral product and are validated for their virus removal capability. Thus, virus removal is a by-product of normal operation. Finally, at the end of the process, steps such as chromatography, filtration or heat may be added to increase overall virus clearance. This strategy has two shortcomings; 1) the virus clearance of these operations may not apply to putative virus that cannot be assayed; and 2) the virus clearance of the process needs to be monitored continually.
Ultrafiltration membranes have been proposed to separate virus from protein in solution. The ideal membrane would retain virus on the basis of its size and allow smaller proteins to pass. Indeed, ultrafiltration membranes are used in the biotechnology industry for this purpose. However, present asymmetric ultrafiltration membranes lack the resolution and reproducibility to perform an optimized virus-protein separation. Typically, asymmetric ultrafiltration membranes that are porous enough to pass economically useful percentages of protein, lack the consistency and high level of virus retention to obtain optimum performance that does not require continuous monitoring and revalidation.
U.S. Pat. No. 4,808,315 describes a hollow fiber membrane with a unique pore structure that is useful in the removal of virus from protein solutions. The membrane is not an asymmetric skinned ultrafilter possessing a surface retention mechanism. Rather it retains virus particles within its structure. It is described as a novel porous hollow fiber membrane which is characterized by such a unique porous structure that the inner and outer membrane surfaces have an in-a-plane average pore diameter of 0.01 to 10 microns and the porous membrane wall has an in-a-plane porosity of not less than 10% measured in every plane perpendicular to a radial direction of the annular cross-section of the hollow fiber membrane, wherein the in-a-plane porosity exhibits at least one minimum value between the inner and outer membrane surfaces.
U.S. Pat. No. 4,824,568 discloses a process for forming an asymmetric skinned membrane on a porous support. The patent does not disclose whether the membrane is useful for the selective removal of virus from a protein-containing solution, nor does it disclose what modifications would be required to obtain a microstructure useful for reproducibly and selectively removing virus particles from protein-containing solutions.
An asymmetric ultrafiltration membrane system that can recover more than 95% of commercially significant proteins and can be validated having a log reduction value of at least about 3 logs of virus particles on the basis of size (retention increasing monotonically as a function of virus particle size) would offer a significant improvement over those available commercially today. This membrane and the system utilizing the membrane could then be used confidently to remove putative virus of any size reproducibly and conveniently without the need for costly monitoring and revalidation.
In addition, such a membrane could be utilized in other applications where it is desired to remove small particles from solution such as in the electronics industry.